Bill Finagin, an aerobatic instructor and air show pilot in Annapolis, MD has taught in the SF260. His feeling is that the SF260 has a very critical wing. Basically this means that the wing is thin, stalls violently, creates tremendous drag at a high angle of attack, and is not for the novice or faint-hearted pilot. In the event of an engine failure on take off you have less than one second to get the nose down since there is not much down elevator authority at low airspeeds and without the air blast from the propeller any hesitation will result in a nose high stall.
This pretty much says it all but unless you experience each event with a knowledge of what is going to happen you will never recognize it. So let us start with the first indication that something is different with the critically thin airfoil.
a. Upon landing for the first time, with my instructor, I brought the plane in to a point 1 foot off of the runway. I was at 90 knots when we crossed the numbers and the speed slowly bled off. We had full flaps and as we approached stall speed I increased backpressure to maintain a constant height just 12 inches off the runway. Suddenly the bottom fell out and the plane impacted the runway with a shock. I found this hard to explain. How could I make such a hard landing when the same technique in my Grumman Tiger would have greased the plane onto the ground. Well, if you look at the thick airfoil on the Grumman Tiger wing and the thin airfoil on the SF260 you can start to appreciate the two factors that were going on. It was six months later, while reviewing for the tenth time the proper care of the landing gear strut, that I realized that the strut did not have enough hydraulic fluid in it to cushion the landing. The SF260 has a trailing link gear that will smooth out the roughest field but it must have enough hydraulic fluid in it to give some delay in compression upon landing. There is a metering valve in the strut and if fluid does not cover it the impact of even the softest landing will only be cushioned by nitrogen, which will compress instantly. With no metering the nitrogen then extends the gear further adding to the impact. The second factor is the critical wing. The SF260 wing will approach a stall in ground effect with no warning except the stall warning horn. At 75 knots the stall warning horn starts to blare away. So you hold that altitude and keep increasing backpressure. In the T-34 the plane will start to settle until it is rolling on the runway. Not the SF260. It will stay at some distance above the runway until it suddenly stalls and when it does it does not settle, it falls out of the sky. The main gear will hit first. This simply means that the nose does not drop, you are flying in a slightly nose high attitude and when the plane stalls it starts to fall. If you are higher the nose would finally drop but not upon landing. So why not fly it onto the ground. One Air force pilot told me that he would use power during landing so he could grease the plane on. Once he learned how to properly service the gear he was able to do all of his landings dead stick.
What helps in a power on landing is that you have the added effect of the slipstream over the elevator. This enables you to keep the nose just a little higher and as the plane starts to settle you can increase backpressure and increase the angle of attack. But the SF260 wing increases drag dramatically with the increased angle of attack and, suddenly you are one foot above the ground in level flight and the drag gets so severe that you have a stall. This is especially bad if you have ballooned just a little and are waiting for the plane to level out and start to settle so that you can sink onto the runway. The SF260 will balloon and then stall while you are getting ready to ease it back onto the runway. The obvious thought might be to just fly the plane onto the runway. The nose gear is weak and this technique will eventually destroy the nose gear.
Denny Sherman used to own Sherman aircraft sales in West Palm Beach. He has sold the business to his son. I had an opportunity to fly with him in 1996, after owning my SF260 for two years. I scared him so badly that he will not fly with me again.
I had tried to demonstrate to Denny that the stall strips create turbulence at high angles of attack and that when this turbulence impacts the elevator it causes severe buffeting.
When I first bought the SF260 I did not realize this and kept complaining of the buffeting that I would experience when at an increased angle of attack. Finally Bill Vitali took me aside and showed me the stall strips. He said that at high angles of attack these created a burble and that these were amplified when they passed over the elevator.
One flight instructor thought this was the elevator trying to stall because it would buffet or even bang when coming out of a loop. The first time I noticed this was just after I bought the plane and tried to do a loop. I was used to looping a L-19 or a cub or champ but the SF260 must be flown around the loop. When I was on the down side of the loop and would pull back there would be a "bang-bang-bang" as the plane would stall, unload then stall again.
To demonstrate the buffeting I would go 135 knots, establish a 60° bank and then pull back to 3 gs and as the SF260 neared a high-speed stall, like the one that is described in the article you sent me. The plane would buffet badly. Seasoned pilots knew what it was but Denny Sherman was scared to death when I did it with him. Denny had just sold the SF260 that had belonged to Bill Knight when I flew with him. He told me I was being reckless and that I was putting 8g's on the plane to make it buffet like that. I showed him the G meter and it only showed 3 Gs as the maximum we had pulled during the flight.
Denny was still so scared from the flight that he would not believe we had only pulled 3 Gs. My personal opinion is that the reason the SF260 has never had an in-flight structural failure is because the wing will stall before it reaches the 6 Gs that it is certified for. Also, the down elevator travel is so limited that the most you can get out of it is -1.5Gs. This is not enough to push over from inverted to an outside ½ loop.
This is one entertaining Email !! By the way, the Falco also has the same stall strips, although I'm not certain that they're the same size as on the Marchetti. Alex